1. Field of the Invention
The present invention relates to a vehicle lamp and more particularly to a vehicle lamp that uses light-emitting diode as a light source thereof.
2. Prior Art
One type of a vehicle lamp uses a light-emitting diode as its light source. A light-emitting diode (LED) has several advantages. Compared to an incandescent bulb, the light-emitting diode is small and lightweight, its electric power consumption is low, and a heat generation amount is small. The light-emitting diode has another advantage. The time period between the electric current is supplied and the highest luminous intensity is reached is short compared to the incandescent bulb.
Accordingly, when the light-emitting diode is employed in a stop lamp for an automobile, it works well in response to the braking operation and thus contributes to a traffic safety.
In addition, since the light-emitting diode is small in heat generation, even when it is disposed in a small-volume light chamber, it has no thermal effect on a lamp body and a lens. As a result, not much consideration is needed to be given to the heat resistance in terms of the materials and the shapes of the lamp body and lens, and also the flexibility in design including selection of the materials is fairly high.
Meanwhile, a so-called high mount stop lamp is a vehicle lamp that uses the above-described advantages of the light emitting diode. The high mount stop lamp is disposed on the inner side of the rear windshield of an automobile.
In such a high mount stop lamp, the lamp chamber, that is defined by a lamp body and a lens covering the front of the lamp body, is generally shallow in the longitudinal direction and long in the lateral direction, and its vertical length (height) is small. Inside the lamp chamber is provided with an LED circuit board that has light-emitting diodes disposed in the lateral direction on its front side, and a lighting circuit is also provided on the LED circuit board.
As described above, with the use of the light-emitting diode as a light source, it is possible to configure a preferable vehicle lamp. In other words, the temperature in the lamp chamber can be prevented from becoming too high even if the lamp chamber is shallow in the longitudinal direction and small in the height. Also, such a vehicle lamp can be installed in the interior of an automobile that has a limited space.
Meanwhile, an electric power supply (a battery) used in an automobile is a 24V- or 12V-battery. Accordingly, a limiting resistor for limiting the current to be supplied to the light-emitting diode is required in a circuit which is for lighting the light-emitting diode.
FIG. 6 shows one example of a circuit a for lighting light-emitting diodes.
In this lighting circuit a, a ground terminal GND as an electric power supply terminal, an electric power supply terminal STOP for a stop, and an electric power supply terminal TAIL for a tail are provided. A light-emitting diode block d formed by connecting a plurality of light-emitting diode groups b in series is connected to an electric power supply line c that connects the terminals GND and STOP. Each of the light-emitting diode groups b is formed by connecting a plurality of light-emitting diodes LED. In addition, a protective circuit e is provided parallel to the light-emitting diode block d. Also, a tail control circuit f for reducing the luminous intensity of the light-emitting diodes LED is disposed between the terminal TAIL and the input side contact point of the light-emitting diode block d. The reference numerals D1 and D2 in FIG. 6 are diodes for preventing backflow.
The thus structured lighting circuit a is provided on a print circuit board (an LED circuit board). Each of the terminals STOP and TAIL and a battery (not shown) are connected by cords g and h, and the terminal GND is grounded using a cord i. Switches (not shown) are provided on the cords g and h.
Each of the resistors RS for current regulation is a surface mounting type resistor, and it is installed on the LED circuit board. All the resistors RS are divided into a plurality of resistor groups j, and each of the resistor groups j is formed by connecting a plurality of resistors RS in parallel. These resistor groups j are provided between the light-emitting diode groups b and connected to the light-emitting diode groups b in series.
The light-emitting diodes LED are mounted on the front surface of the LED circuit board; in other words, they are provided on the surface that faces the lens of the stop lamp. The resistors RS, on the other hand, are mounted on the rear surface of the LED circuit board. When a transparent and colorless clear lens is used, the inside of the lamp chamber can be seen to its detail. Therefore, in consideration of appearance, it is preferable that the resistors RS be provided on the rear side of the LED circuit board. It is also preferable to use surface mounting type resistors that have terminals not projecting from the front surface.
Despite the structure described above, since the surface mounting type resistors RS do not have a large electric capacity, the problem is that the number of the resistors RS to be mounted on the LED circuit board tends to be large.
When a large number of the resistors RS are mounted on the LED circuit board, another problem rises. The size of the LED circuit board needs to be large, and this hinders the size reduction of the lamp.
In addition, the resistors RS generate heat and the number thereof becomes larger, the entire heat generation amount on the LED circuit board becomes larger, thus reducing the luminous intensity of the light-emitting diode LED by the heat. Accordingly, it is necessary to increase the number of light-emitting diode LED, which hinders the size reduction of the lamp.
Further, when the resistors RS are mounted on the LED circuit board and the number thereof becomes larger, the parts cost and mounting cost increases, and the cost of the lamp also increases.
Accordingly, the object of the present invention is to provide a lamp structure in which a current limiting resistor is provided independently of an LED circuit board, thus resolving various problems that result from current limiting resistors mounted on the LED circuit board.
The above object is accomplished by a unique structure for a vehicle lamp that includes a lamp chamber defined by a lamp body and a lens that covers the front of the lamp body, an LED circuit board which is disposed inside the lamp chamber and on which a light-emitting diode is mounted and the lighting circuit is formed, and a cord for connecting the LED circuit board to an electric power supply; and in the present invention, a limiting resistor for limiting the supply current for the light-emitting diode is provided on a part of the cord that is located outside the lamp chamber.
In the above structure of the present invention, the limiting resistor is not mounted on the LED circuit board. Accordingly, it is possible to avoid the size increase of the LED circuit board and avoid the efficiency drop of the light-emitting diode that is caused by the heat on the LED circuit board.
Also, since the limiting resistor is provided on the cord, it is possible to use a limiting resistor that has a large capacity, and the cost for providing the limiting resistor can be reduced, and thus the costs of the lamps can be reduced.